Pulsed Supersonic Jet with Local High Speed Valve

ABSTRACT

A pulsed supersonic jet excavator uses a short duration blast of air to excavate using a minimum amount of air and a minimum reaction force. This device uses a fast acting valve to create a jet of air that lasts about as long as it takes to develop. The result is a device that works much more efficiently than existing air jet excavators. This device can be mounted on a small robot and allow it to dig, whereas a normal backhoe type excavator would just lift the robot when attempting to dig in packed earth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationNo. 61/327,832, filed 2010 Apr. 26 by the present inventor.

BACKGROUND

This application relates to excavators, particularly to portablepneumatic excavators.

PRIOR ART

Excavators which use a jet of air are well known, and they may be usedto excavate mines, gas lines and such. These devices may be pulsed bythe operator, or they may be pulsed by valves located in the handle, asin U.S. Pat. No. 5,966,847. These devices are cumbersome and bulkybecause they use powerful air compressors and large quantities of air.Prior devices waste air while building up to a supersonic jet andtapering down to zero pressure. The pressure during the rise and falltime is not sufficient to dig earth. In a previous application20090044372, the inventor describes a device for cleaning surfaces whichuse supersonic jets. During the development of that device I discoveredthat short pulses of air are just as effective as long ones. Therefore adevice which incorporates a fast acting valve located nearby a De Lavalnozzle will provide the best excavation rate for the least amount of airconsumption.

SUMMARY

In accordance with one embodiment an excavator includes a source ofcompressed air or gas such as a tank or compressor, an air conduitleading to a pulse jet, a nozzle to accelerate the air to maximumvelocity, and at least one valve to let the air out through the nozzlein a sharp pulse. In some embodiments an electric or pneumatic circuitcontrols the operation of the valve or valves.

At present I believe the pressure of the air should be approximately 300psi in order to create a supersonic jet of reasonable length, such thatthe air reaches the surface and recompresses in order to provide themaximum pressurization and shear force on the earth, but higher or lowerpressures are also satisfactory. A pressure regulator may be used inbetween the tank or compressor and the valve. A heater or heat exchangermay be used upstream of the valve in order to keep the specific volumeof the air at a high level.

I have found that the digging action occurs as the air pressure rises inproximity to the ground during the initial formation of the supersonicjet. The air pressure before the supersonic jet formation is notsufficient to dig. In one embodiment, the pulse duration is only longenough for the jet to form. This conserves air consumption. This isachieved by using a fast-acting valve which is close coupled to thenozzle. The time required to pressurize the nozzle ahead of the systemis minimized. Such a valve is described in U.S. Pat. No. 5,271,226. Thistechnology is common in the art of cold gas thrusters. At present I havefound that a Marotta MV78C valve operates most efficiently, but otherfast-acting valves are also satisfactory. The Marotta valve is anaerospace poppet valve which is actuated by a small balanced pilotvalve.

In some circumstances, the flow of air to the valve may be limited. Inthis case a reservoir of air may be located close to the valve. Thevalve may be designed with hysteresis to open at a given pressure andclose at a lower pressure. For example the valve may open a 300 psi andclose at 100 psi. This type of valve converts a steady low flow of airto a pulsatile flow of air with a tapering off of pressure in eachpulse. This allows the device to loosen soil with the initial pressurepulse and then blow it away with a lower pressure, thereby achievingeffective excavation with a minimum of air consumption.

Another embodiment has a continuous supply of air to the valve andnozzle, an air storage chamber near the nozzle, and a dump valve whichopens when the pressure in said chamber reaches a given value. Thisembodiment allows for the use of a smaller air conduit and pressureregulator leading to the pulse jet.

In some other embodiments, the air is heated before it reaches thenozzle. This has the advantage of generating more pulses for a givenamount of air. It also helps counteract the cooling of the air whichoccurs as an air tank is consumed. This may be accomplished by means ofa heat exchanger which uses the outside air, or a fuel driven heater toheat the air. It is well known that air cools when it expands through asupersonic nozzle, so the air can be heated up above ambient temperatureand still result in a pulse jet that is at ambient temperature.

The nozzle is a standard De Laval type, which accelerates the air usinga contraction and expansion section. The exit area can be determinedbased on the upstream pressure and local ambient pressures. This type ofnozzle is common in rocket engines and the equations to design them arewell known in the art.

In order to achieve the required short pulse, the valve must actuatevery quickly, or the air supply must be limited. In some embodiments theair supply will be from a pressurized tank of air at 2000-5000 psi. Insome embodiments in order to get the most energy out of the air, a valvewhich operates very quickly is used to utilize the full pressure of thetank. In some embodiments, adjusting the on time of the valve controlsthe impulse generated. This way, all the energy of the tank is used.Another possible scenario is to use a spring loaded accumulator inbetween the two valves. That way the air pressure does not need to taperoff and more of the energy in the air can be utilized. In someembodiments the pressure in the air storage container is controlled bycontrolling the on time of the inlet valve. In this way, and adjustablepressure pulse is delivered.

ADVANTAGES

Several advantages of one or more aspects are to provide an excavatorthat is more efficient, more portable, inexpensive, has less reactionforce, and provides more effective excavation with high pressure andless air consumption. Other advantages of one or more aspects are toprovide an excavator that may use a local compressed air source tank andis not tethered to a compressor. Other advantages of one or more aspectsare to provide a robotically controlled excavator wherein the robot issmall and light-weight, and could not readily operate any other type ofdigging tool. Other advantages of one or more aspects will be apparentfrom a consideration of the drawings and ensuing description.

DRAWINGS Figures

In the drawings, closely related figures have the same number butdifferent alphabetic suffixes.

FIG. 1 is a schematic drawing of one embodiment with a local air chamberin the shape of a cylinder.

FIG. 2 is a drawing of a quick opening poppet valve in the closedposition.

FIG. 3 is a drawing of a quick opening poppet valve in the openposition.

FIGS. 4A to 4C show the dimensions determined for a given nozzle throatdiameter.

FIGS. 5A and 5B show the results of one test of the stagnation pressureand duration of each pulse of air.

FIG. 6 shows a schematic of the logic used to control one embodimentwith two valves.

FIG. 7 shows a graphical representation of the air conserved with asharp supersonic pulse instead of a gradual pressure pulse.

FIG. 8 shows one embodiment including a pulsed supersonic jet with localhigh-speed valve attached to a robot.

FIG. 9 shows one embodiment including a pulsed supersonic jet with localhigh-speed valve and local video camera

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing of one embodiment of the device. Thenozzle 14 and a valve 13 are located is close proximity so that as thevalve opens, little air is wasted filling the duct in between the nozzleand the valve. A local air chamber 12 holds a volume of air to bedispensed. In operation the prefill valve 11 lets high-pressure air intolocal air chamber 12. Valve 11 then shuts and valve 13 opens, dumpingthe air through the nozzle. In one embodiment, operation of the valvesis controlled by an electric or pneumatic circuit. In some embodiments,the high pressure air is be supplied by a compressor or a tank ofcompressed air upstream of conduit 10.

In FIG. 2 a nozzle and valve combination is illustrated. This includes avolume of air in toroidal prefill chamber 21 in close proximity to valveseat 27. The poppet 25 keeps the valve closed until the pressure appliedover the outer ring 23 overcomes the pressure in dome load compartment22, which is augmented in some embodiments by a spring (not shown). Oncethe pressure in chamber 21 pushes the poppet 25 up, the air then fillsin the entire area 28 under the poppet, forcing it up. The throat 26 ofthe nozzle becomes the flow limiting feature. As the restricted inletflow is not sufficient to maintain the high pressure continuously, thepressure behind the nozzle 14 falls to a lower pressure, such as 100psi, at that point, the pressure in compartment 22 is enough to push thepoppet 25 back on the seat 27. At that point the valve closes and doesnot reopen until the pressure rises to 300 psi in prefill chamber 21.The poppet may be limited to a purely up and down movement by poppetguide 24.

In FIG. 3, the same valve is shown in the open position. In thisposition the pressure in chamber 21 is applied to the whole diaphragm 23and poppet 25.

FIG. 4 shows the nozzle dimensions for one embodiment for ideallyexpanded supersonic flow. The nozzle exit diameter for various feedpressures was determined using adiabatic and isentropic relations forsupersonic flow. The calculations assume air as the working fluid and athroat diameter of 0.635 cm (0.25 inch). My present nozzle has a 0.635cm (0.25 inch) throat diameter, a 15° cone angle, and a 1.105 cm (0.435inch) diameter exit nozzle, but other nozzle dimensions are alsoacceptable.

FIG. 5 shows a stagnation pressure test of one embodiment which includeda single Marotta MV78C valve. The valve was commanded to open for 40 msand the resulting stagnation pressure pulse width was 63 ms, but otherpulse durations are also satisfactory.

FIG. 6 shows the control sequence of one embodiment including two valvesin order to produce a short pulse width pressure burst. The fill length30 is the period of time the upstream valve 11 is open. The fire delay32 is the period of time that both valves remain closed. The firinglength 34 is the period of time the downstream valve 13 is open andexhausting the air in the volume between the two valves. The delay untilfill 36 is the period of time from when the downstream valve 13 isclosed and the upstream valve 11 is opened again.

FIG. 7 shows a graphic representation of a sharp supersonic pulse 44 anda gradual pressure rise and fall 42. The pressure where digging occursis at pressure level 38 and above. The sharp burst 44 provided with thelocal high speed valve uses nearly all of the air in the digging region38. The gradual pressure pulse 42 wastes air during time spent in thelow pressure area 40 where no digging occurs.

FIG. 8 shows one embodiment of the pulsed supersonic nozzle 14 withlocal high-speed valve 13 attached to a robot 46. In this embodiment, atank of compressed air 48 supplies the high pressure air, but acompressor would also be satisfactory. In this embodiment, thehigh-speed valve 13 is controlled by a pneumatic circuit 50, but anelectric circuit would also be satisfactory.

FIG. 9 shows one embodiment of the pulsed supersonic nozzle 14 withlocal high-speed valve 13 attached to a robot 46 including a videocamera 52.

I claim:
 1. A pulsed supersonic jet excavator consisting of A supersonicnozzle including a converging portion and a diverging portion. Ahigh-speed valve located within 10 nozzle diameters of said nozzle, Saidnozzle and valve being attached to a remotely controlled manipulationmeans.
 2. The device as in claim 1, including a local air storagechamber which is periodically dumped through the valve
 3. The device asin claim 1, including a heater to heat the air.
 4. The device as inclaim 1, including a poppet valve wherein the downstream pressuremotivates the valve towards an open position.
 5. The device as in claim1, including a robot to carry said device.
 6. The device as in claim 1,including a video camera to feed data to a remote location.
 7. Thedevice as in claim 1, including an attachment to a vehicle.
 8. Thedevice as in claim 1, with including a metal detector to detect buriedobjects.
 9. The device as in claim 1, wherein the volume of the fluidconnection between said high speed valve and said nozzle is less than 5times the volume of said nozzle
 10. The device as in claim 2, includinga pressure transducer to measure the pressure in said storage chamber.11. The device as in claim 1, Said valve open duration being less than10 times to the time required for the supersonic jet to develop to itsfull supersonic length
 12. The device as in claim 1, with a local videocamera with positioning means to observer the excavation.